Should I put my dog down?

My dog is a 17 year old cockerspaniel poodle mix, and she is doing amazingly well. Unfortunately, I can't afford to have her looked at by a vet, and just a few days ago she started limping on her front left paw really bad. She still seems spunky and full of life, but she can hardly see some days and I think she may have a cancerous growth on her stomach. I really don't want to put her down if she is happy and full of life, but if she is suffering, I'd like to end that for her. I'd love to take her to a vet to get examined, but if I do have to put her down, I wouldn't be able to because of all the costs. Please help.